Quality evaluation for wireless communication networks

ABSTRACT

Systems and methods for evaluating wireless network quality. A metric provided by embodiments of the present invention relies on information that is relatively easy to collect, can be very efficiently computed, and yet provides a realistic estimate of likely wireless network performance. In one implementation, the input includes path loss data and access point transmitter power level and frequency settings. A capacity indicator is computed for each client and each access point. A data rate indicator is computed for each client location. The traffic load is computed for each access point. Based on these computed indicators, a bidirectional client throughput can be computed for each client and a combined metric can be determined for the network as a whole.

BACKGROUND OF THE INVENTION

The present invention relates to wireless communications and moreparticularly to systems and methods for evaluating network quality.

Wireless communication networks, for example wireless communicationnetworks providing network access in a building or on a campus, arehighly complex systems that serve a multitude of client devices usingmultiple access points. The network planner seeks to provide ubiquitouscoverage, high throughput, and relatively even loading on the accesspoints. There are many network parameters that may be adjusted inseeking to achieve this ideal. These parameters include placement of theaccess points, frequencies of operations of the access points,transmitter power levels, modulation rates, etc. Algorithms have beendeveloped that seek to optimize these parameters to achieve the bestnetwork service possible.

To determine whether a particular combination of parameter values isoptimal in any sense or better than some other combination ofparameters, it is necessary to devise a metric for assessing networkquality. In the course of performing either a manual or automaticallyoperated optimization algorithm, such a metric will have to be evaluatednumerous times. A requirement thus emerges for a network quality metricthat requires only information that is relatively easy to assemble andinput and can be evaluated relatively quickly for a given set of networkparameters values, but yet provides a realistic estimate of likelyperformance of the network that corresponds to the user experience.

Previous network planning tools and their associated metrics have beendeveloped in the context of cellular telephone systems. By contrast, thewireless local area network (LAN) applications have differentcharacteristics that change the nature of the network evaluationproblem. As compared to LANs, cellular networks are outdoors, operateover longer ranges, typically operate at lower carrier to interferenceratios, and use very different methods of media access control. Thequality metrics developed in the cellular telephone context are notapplicable to wireless LANs.

What is needed are new systems and methods for evaluating the quality ofwireless networks including wireless LANs.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide systems and methods forevaluating wireless network quality. A metric provided by embodiments ofthe present invention relies on information that is relatively easy tocollect, can be very efficiently computed, and yet provides a realisticestimate of likely wireless network performance. In one implementation,the input includes path loss data and access point transmitter powerlevel and frequency settings. A capacity indicator is computed for eachclient and each access point. A data rate indicator is computed for eachclient location. The traffic load is computed for each access point.Based on these computed indicators, a bidirectional client throughputcan be computed for each client and a combined metric can be determinedfor the network as a whole.

One embodiment of the present invention provides a method of assessingcommunication quality in a wireless network comprising a plurality ofaccess points. The method includes: receiving as input path lossinformation indicating path losses between a selected client of saidwireless network and said access points, based on said path lossinformation, determining a capacity indicator that estimatescommunication impairment for said client due to contention or collision,based on said path loss information, determining a data rate indicatorthat estimates an achievable data rate for communication by saidselected client, determining a cell loading indicator that estimatescommunication impairment due to overloading of a cell occupied by saidselected client, and, based on said capacity indicator, said data rateindicator, and said cell loading indicator, determining a clientthroughput.

Further understanding of the nature and advantages of the inventionsherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart describing steps of evaluating communicationquality for a selected client according to one embodiment of the presentinvention.

FIG. 2 depicts access point contention with other access points asevaluated by embodiments of the present invention.

FIG. 3 depicts access point contention with clients as evaluated byembodiments of the present invention.

FIG. 4 depicts access point collision with other access points asevaluated by embodiments of the present invention.

FIG. 5 depicts client contention with access points other than an accesspoint associated with the client as evaluated by embodiments of thepresent invention.

FIG. 6 depicts client collisions with access points other than an accesspoint associated with the client as evaluated by embodiments of thepresent invention.

FIG. 7A depicts coverage metrics for different variants of the 802.11standard according to one embodiment of the present invention.

FIGS. 7B-7D depict data rate metrics for different variants of the802.11 standard according to one embodiment of the present invention.

FIG. 8 depicts a cell loading metric according to one embodiment of thepresent invention.

FIG. 9 depicts a computer system useful in implementing embodiments ofthe present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention will be described with reference to arepresentative wireless network that employs one of the IEEE 802.11standards such as, e.g., 802.11a, 802.11b, 802.11g, or currentlyenvisioned standards such as 802.11n. All of the documents definingthese standards are incorporated herein by reference in their entiretyfor all purposes. In the example discussed herein, a region to becovered by a wireless network is divided into cells with each cellhaving an access point. Clients are associated with a particular accesspoint and can communicate to and from the network via that access point.

When a radio plan is developed for wireless LAN, the operator hascontrol of at least two different parameters for each access point: thechannel assignment and transmit power. The operator may also havecontrol of other parameters such as the threshold for packet detectionand allowed data rates.

The goal of a radio planning algorithm is to find the access pointsettings that provide the best possible solution. The ideal solution isone that provides high overall network throughput, complete coverage ofa particular space, and relatively even loading on the access points. Acritical step in this process is evaluation of the quality of eachpossible solution. The quality evaluation should preferably take intoaccount the following factors:

1. The channel of each access point and associated clients.

2. The transmit power of each access point and associated clients.

3. The sensitivity and required carrier to interference ratio for eachpossible physical layer data rate.

4. Contention and collision between access points and clients thatoccupy the same channel and that occupy adjacent channels.

5. Interference from access points that are not controlled by theoperator.

6. The traffic load imposed on each access point by its clients.

7. The physical space to be covered.

8. The propagation characteristics of that physical space.

9. The data modes enabled at each access point.

Embodiments of the present invention provide a metric that takes allthese factors into account and quickly converts them into a measure ofoverall “goodness.” A radio planning algorithm can then efficientlysearch through different possible combinations of access point settingsand find a globally optimal, or at least a very good, solution.

FIG. 1 is a flow chart describing steps of evaluating communicationquality for the communication network according to one embodiment of thepresent invention. At step 102, the metric evaluation procedure receivesinput specifying path losses and access point settings. The path lossvalues indicate the path attenuation from a series of walkabout pointsto each access point as well as the attenuations between access points.Some of the walkabout points will correspond to actual client locationsor be used as proxies for client locations in the calculations thatfollow. The path loss values are preferably based on actual measurementsrather than on propagation modeling. The access point settings includethe channels of operation. For example, in one implementation, thechannel set (1, 6, 11) is specified for each access point. It is alsopossible for the input to specify a particular channel set among severalselections including, e.g. (1, 6, 11), (1, 4, 8, 11), or (1, 4, 6, 8,11). The access point settings may also include a list of allowed datarates since access points may be configured to operate only in a subsetof possible data rate modes.

A series of steps following step 102 are performed iteratively for eachclient. The term “client” as used herein is taken to also includewalkabout locations taken as proxies for clients. Step 104 determines abi-directional capacity indicator for a selected client. Capacity asdefined in this context as how readily an access point can transmit datadownstream to clients, or conversely how readily a client can transmitinformation upstream to the access point. Effectively, thebi-directional capacity indicator measures impairment due to likelycontention or collision situations. Details of computing thebi-directional capacity for a selected client are described in detailbelow. The computation of the bi-directional capacity indicatorincorporates an upstream capacity computation for the client and adownstream capacity computation for the access point the client isassociated with.

A step 106 determines a data rate indicator for the selected client. Thereceived signal strength is mapped into a rate of data transfer betweenthe client and the access point. Each possible data rate has a signallevel above which data can be transferred reliably. The received signalstrength is mapped to a physical layer data rate using a lookup table.That physical layer data rate is then converted into a MAC layer datarate using a lookup table such as the one that follows. Modulation DataTypical Rx MAC Mode Rate Sensitivity (typical) Throughput 802.11b  1Mb/s −92 dBm .75 Mb/s  2 Mb/s −86 dBm 1.2 Mb/s 5.5 Mb/s  −84 dBm 2.3Mb/s 11 Mb/s −77 dBm 3.2 Mb/s 802.11a  6 Mb/s −88 dBm 3.4 Mb/s  9 Mb/s−79 dBm 4.6 Mb/s 12 Mb/s −81 dBm 5.4 Mb/s 18 Mb/s −78 dBm 6.7 Mb/s 24Mb/s −76 dBm 7.8 Mb/s 36 Mb/s −69 dBm   9 Mb/s 48 Mb/s −64 dBm 9.7 Mb/s54 Mb/s −60 dBm 10.1 Mb/s 

FIGS. 7B-7D graphically illustrate the relationships between receiversensitivities and data rates for 802.11a, 802.11b, and 802.11 g,respectively. The mapping of the physical layer data rate into the MAClayer data rate depends on the average packet size and the MAC protocolbeing used. Therefore the network performance can be optimized for voice(short 100 to 200 byte packets) or for large TCP-IP transfers (1536 bytepackets). The above table and FIGS. 7B-7D are for a typical packet mixwith a mean packet size of 364 bytes.

A step 108 determines a cell loading indicator for the selected client.Cell loading actually needs to be determined only once for each accesspoint so it will be understood that the cell loading indicator for aclient is in fact a cell loading indicator of the access point to whichit is associated. The cell loading indicator accounts for a throughputdrop that results when too many clients are associated to a singleaccess point. The user of the evaluation procedure defines the maximumnumber of clients that can be associated to a single access pointwithout performance degradation. Up to that maximum number, nodegradation is experienced while beyond that number, the cell loadingmetric falls off proportionally to 1/(number of associated clients).Further details of cell loading are explained below.

The capacity, data rate, and cell loading indicators are used to providea measure of the data throughput of each client. At step 110 determinesa scaled client capacity for a selected client. The metrics are combinedas follows:Client Throughput=Client Bidirectional Capacity Indicator*Client DataRate*Cell Loading Indicator.

The client throughput provides an estimate of the mean rate of datatransfer between the client and its access point. The reciprocalprovides a measure of the amount of time it will take to transfer largedata records to and from a particular client.

A step 112 tests whether the calculations of steps 104-110 have beendone for all clients in the network. If there are further clients forwhich to compute the appropriate indicators, step 114 picks the nextclient as the selected client and then execution returns to step 104. Ifscaled client capacity has been determined for all of the clients, thenthe metric computation reaches step 116 where a total combined metricfor the network is determined. The combined quality metric is preferablydefined as:$\frac{1}{\sum\limits_{{all}\quad{clients}}{{1/{client}}\quad{throughput}}}$

The above combined metric is not exactly the same as the total networkcapacity. The combined metric gives more weight to client locations withpoor performance than those with good performance. A network where 90%of the clients can receive 11 Mbps and 10% of the clients can receivenothing is penalized as compared to a network where 80% of the clientsreceive 11 Mbps, 10% receive 5.5 Mbps, and 10% receive 1 Mbps.Alternatively, a total network capacity may be determined as a mean ofall of the scaled client throughputs times the total number of accesspoints in the network:

Capacity Details

Capacity is defined for each access point, for each client location, andfor the entire network. The evaluation procedure relies on assumptionsas to the fraction of time that the fully loaded wireless mediumtransmits successfully in the uplink and downlink. Representative valuesare P_(U)=0.2 (probability that a transmission on that link will beupstream) and P_(D)=0.8 (probability that a transmission on that linkwill be downstream). From the viewpoint of capacity, the ideal is asingle access point and a few clients operating with no potentialco-channel or adjacent-channel interference. Such a situation will yielda capacity measure of 1. Interference from other cells will lower theexpected capacity for that cell to some value less than 1. The metricpenalizes capacity when stations experience contention or collision.Capacity computations depend on received signal strengths. The receivedsignal strengths are determined based on the transmit power and pathlosses that were input to the algorithm.

There are theoretically 9 different types of contention and collisionthat could occur within a cell. There are:

1. External access points contending with the access point attempting totransmit downstream.

2. External access point colliding with the access point attempting totransmit downstream.

3. External access point contending with a client attempting to transmitupstream.

4. External access point colliding with a client attempting to transmitupstream.

5. External client contending with an access point attempting totransmit downstream.

6. External client colliding with an access point attempting to transmitdownstream.

7. External client contending with a client attempting to transmitupstream.

8. External client colliding with a client attempting to transmitupstream.

9. Client collides with another client in the same cell.

To alleviate the need for cumbersome client-to-client path lossmeasurements, the presently described evaluation procedure only takesinto account the first 5 types of contention and collision. The use ofthe scaling factors P_(U) and P_(D) within the capacity calculationsallows results based on only the first 5 types of contention andcollision to serve as a realistic estimate of the desired capacityindicator.

First let us consider the downstream capacity of an access point. Thedownstream capacity of an access point is calculated as its ability totransmit downstream data in the presence of interference from otheraccess points and clients from other cells. The access point capacity isexpressed as a quotient where the numerator is always 1. In an idealcase, the denominator is also 1, but co-channel interference from othercells will increase the value of the denominator. As will be shown, thedenominator will be equal to 1 plus the sum of various degradationindicators.

The capacity calculation including the determination of variousdegradation indicators will be discussed with reference to a specificexample. FIG. 2 shows access point contention with other access points.All 5 access points operate on the same channel and the receiversensitivity for the minimum data rate mode is −85 dBm. The arrows showthe received signal strengths at AP₀ for co-channel transmission by theother access points. The received signal levels are derived from thepath losses and transmission powers that were input to the evaluationprocedure. Signals transmitted by AP₁, AP₃, and AP₄ are all at −80 dBm,5 dB above the receiver sensitivity of AP₀. Since AP₀ will hear thesetransmission before it attempts to transmit, AP₀ will not transmit whenany of these three access points are transmitting. By contrast, signalstransmitted from AP₂ arrive at AP₀ at −90 dBm, below the receiversensitivity threshold. Therefore, contention with AP₂ will not degradethe downstream throughput. The degradation indicator for this type ofcontention is computed to be:P_(d)*No_AP_Contend

-   -   where P_(d) is the probability of the contending access point        wants to transmit, nominally set to 0.8; and    -   No_AP_Contend=the number of transmitting access points that can        be received by the access point of interest, 3 in our example.

In this example, the degradation caused by the other access points is0.8*3=2.4.

FIG. 3 shows downstream traffic degradation due to contention withclients associated with other access points. In this example, L₁₀ andL₁₁ are associated with AP_(l), L₂₀ and L₂₁ are associated with AP₂,L₃₀, L₃₁, and L₃₂ are associated with AP₃, and L₄₀ and L₄₁ areassociatde with AP₄. The sensitivity of AP₀ is −85 dBm so transmissionsfrom L₁₀, L₁₁, L₃₁, L₄₀, L₄₁ will cause AP₀ to wait to transmit itsdownstream data, thus degrading the quality of its downstream link. Thequantitative measure of degradation caused by each potentially collidingclient is calculated as:P_(U)/Number of clients in the same cell.

So for P_(U)=0.2, the contending clients contribute as follows:

-   -   L₁₀=0.2/2=0.1    -   L₁₁=0.2/2=0.1    -   L₂₀=0, No Contention    -   L₂₁=0, No Contention    -   L₃₀=0, No Contention    -   L₃₁=0.2/3=0.067    -   L₃₂=0, No Contention    -   L₄₀=0.2/2=0.1    -   L₄₁=0.2/2=0.1

The sum of all the degradations caused by clients contending with AP₀ is0.467.

FIG. 4 depicts access points colliding with other access points. In FIG.2, it was shown that AP₀ would contend with AP₁, AP₃, and AP₄ for thechannel, but it would not contend with AP₂ because the signal from AP₂was too weak to be detected by AP₀. Therefore it is possible that AP₂could be transmitting simultaneously with AP₀ since AP₀ will not know todelay its transmission. If the signal from AP₂ is strong enough tocorrupt reception at the clients associated with AP₀, AP₂'stransmissions will potentially collide with downstream traffic from AP₀.The potential for collision is based on the needed carrier tointerference ratio. This is determined by first calculating the receivedsignal strengths at the access point and the client, then determiningthe physical layer data rate and required carrier to interference ratioby reference to a look-up table.

Referring now to FIG. 4, if the clients L₀₀ and L₀₁ are operating in adata mode that requires 15 dB carrier to interference ratio, L₀₀ willexperience collisions from AP₂ while L₀₁ will not. The first client, L₀₀receives a signal of −80 dBm from AP₀ and it receives an interferingsignal of −80 dBm from AP₂. The carrier to interference ratio istherefore 0 dB, and L₀₀ will therefore experience a collision. Theclient L₀₁ receives a signal of −80 dBm from AP₀ and an interferingsignal of −100 dBm from AP₂, resulting in a carrier to interferenceratio of 20 dB, sufficient to avoid a collision.

The degradation caused by these access point collisions from anotheraccess point is calculated as follows:$2*{\sum\limits_{{other}\quad{access}\quad{points}}\frac{\begin{matrix}{{{number}\quad{of}\quad{clients}\quad{experiencing}}\quad} \\{{collisions}\quad{from}\quad{other}\quad{access}\quad{points}}\end{matrix}}{{number}\quad{of}\quad{clients}\quad{in}\quad{cell}}}$

The summation is taken over all access points other than the accesspoint whose capacity is being measured. In this example, there is onlyone access point causing a collision, so the total degradation is2*(1/2)=1.

The total access point capacity is then computed as follows:Numerator=1Denominator=(1+Degradation due to access point to access pointcontention+Degradation due to access point to clientcontention+Degradation due to access point to access point collisions)

In this example, the capacity would be:Numerator=1Denominator=1+2.4+0.467+1=4.867Access point capacity=1/4.867=0.205

Contention and collision from other cells will also cause a reduction inthe upstream capacity of each client. Upstream client capacity can bedegraded by contention from other access points as well as collisionfrom other access points. Client contention from other access pointsoccurs when signals transmitted from other cells arrive at the clientand lead the client to believe its channel is busy, causing the clientto delay transmission. Client collision from other access points iscaused when signals transmitted from access points in other cells arriveat sufficiently weak levels such that the client transmitssimultaneously, however, the carrier to interference ratio at theclient's associated access point is too low for successful data recoverythere. Similar to the access point computation, the client upstreamcapacity computation employs a ratio where the numerator is one and thedenominator is one plus a sum of degradation indicators.

FIG. 5 depicts client contention with other access points. The client atlocation L₀₀ wants to transmit data to AP₀ however, it can detectsignals transmitted from AP₁, AP₂, and AP₃. It cannot hear signalstransmitted from AP₄. Whenever AP₁, AP₂, and AP₃ are transmitting, L₀₀delays transmission. The degradation caused by contention from otheraccess points is evaluated to be equal to the number of access pointsfrom other cells that can be detected at the client. In this example,for L₀₀, the degradation is 3.

FIG. 6 depicts client collisions with access points other than the oneit is associated to. Client L₀₀ hears signals transmitted from AP₁, AP₂,and AP₃ so it will delay transmission. However, L₀₀ will not hearsignals transmitted from AP₄ so there is a potential for a collision.When client L₀₀ transmits to AP₀, the signal arrives at −80 dBm. If AP₄transmit simultaneously, the carrier to interference ratio for thereceived client signal is 0 dB, insufficient for successful datarecovery. The indicator for this type of degradation is computed to be 2multiplied by the number of access points capable of causing acollision. An access point is capable of causing a collision if thesignal from that access point received at the client's associated accesspoint causes the received client signal carrier to interference ratio tofall below the threshold necessary for accurate reception. In thisexample, this expression is equal to 2 since there is one such accesspoint capable of causing a collision.

The total upstream capacity for a client is calculated as follows:Numerator=1Denominator=1+Degradation caused by contention with out-of-cell accesspoints+Degradation caused by collisions with out-of-cell access points.

In this example:Numerator=1Denominator=1+3+2=6Total upstream client capacity=⅙ or 0.167.

The total bidirectional client capacity is then:Associated Access Point Capacity*P_(d)+Client Upstream capacity*P_(u)

Where P_(d) is nominally 0.8 and P_(u) is nominally 0.2. In thisexample, the result is 0.1974. This is the value that is used incomputing the scaled client capacity at step 110.

Cell capacity=access point capacity*P_(d)+mean client capacity*P_(u).The mean client capacity is the average upstream client capacity for theclients associated with the access point of a cell.

Cell Loading

Cell loading is a measure of degradation caused by an excessive numberof clients in a cell potentially contending for the same channel. Theexact number of clients that can successfully share a channel in a celldepends on separately generated usage models. A parameter generated bysuch a usage model is max_clients which is the maximum number of clientsin a cell before performance suffers as determined by the usage model.An additional parameter to be entered by the operator is mean_clientswhich is equal to the average number of clients in each cell.

First, the number of clients in each cell is estimated by:EST_CLIENTS=(number of walkabout points in cell/total number ofwalkabout points)*mean_clients. The capacity scaling factor due toovercrowding on an access point is then calculated as:Cell_Loading_factor=max_clients/max(max_clients, est_clients)

FIG. 8 depicts how the cell loading factor varies as the number ofclients increases. When max_client is set to 30, there is no penaltyuntil the number of clients exceeds 30. After that, the cell loadingfactor decreases proportionately to (1/number of clients).

It will be seen then that the various interference degradationsincluding contention and collision are quickly converted into estimatesof how readily information can be transferred to and from a particularaccess point and client. The overall metric, by being a product ofcapacity, data rate, and cell loading, takes into account interferencefrom other cells, the strength of received signals, and contentionwithin the cell.

The adjustment of wireless network operation parameters involves atradeoff between two factors. As power increases, the ability of eachaccess point to transfer data at the highest possible data rateimproves. However, interference between cells operating on the samechannel also increases. The metric of network quality provided byembodiments of the present invention facilitates finding the optimalpoint in that tradeoff. Since the metric is calculated readily usingmeasured data, operation of the parameter search algorithm isfacilitated. Also, by use of this metric, optical network performancewill be obtained since what is being minimized is the meantime for datatransfer to and from the clients.

Significant advantages are provided over planning tools that rely onpropagation modeling. No propagation model is ever 100% accurate and atypical RMS error for path loss models is 5 to 10 dB. A relatively smallpath loss error can make the difference between whether two accesspoints contend for a channel or not. By employing real measured pathloss data, uncertainty in evaluating the likelihood of contention orcollision is minimized.

The example capacity calculation described above dealt with co-channelinterference. Embodiments of the present invention may also deal withadjacent channel interference that causes contention and collisions. Asuitable additional attenuation factor may be added to path losses foradjacent channel transmitters to determine whether contention orcollision is possible from a given transmitter.

Coverage

It may also be useful to compute an additional coverage metric. Coverageis defined in this context as a unitless measure of available physicallayer data rate between the client and the access point to which it isassociated under the relevant operative protocols. When the signalstrength from the access point to the selected client is lower then thesensitivity of the minimum configured data rate mode, no information canbe transmitted from the access point to the client so the coveragemetric is zero. When a signal strength from the access point to theselected client is several dB above the sensitivity of the maximumconfigure data rate mode, it is very likely that information can betransmitted at the highest data rate, so the coverage metric is one.Signal strengths between those two levels are mapped into a coveragemetric by a linear function. Further details of computation of thecoverage indicator will now be given.

The coverage metric uses the signal strength from the access points thatproduce the strongest and second strongest received signals at theselected client. This takes into account that in a heavily loadednetwork, association requests may sometimes be denied to clients,causing an association request to another access point. The coveragemetrics are calculated as follows:

1. Calculate the received signal strength from every access point to theclient.

2. Find the strongest received signal (r₁) and the second strongestreceived signal (r₂).

3. Convert the signal strengths (r₁) and (r₂) into coverage metricsusing the following function:F(r)=0.0 for r<R_(SensMin)0.7 (r−R _(SensMin))/(R _(SensMax)+10−R _(SensMin)), R _(SensMin) <r<R_(SensMax)+100.7, R _(SensMax)+10<r

-   -   where R_(SensMin) is the sensitivity for the minimum data rate        mode and R_(SensMax) is chosen to be a figure several dB above        the sensitivity for the maximum configured data rate mode. An        802.11b network will have R_(SensMin)=−94 dBm and        R_(SensMax)=−85 dBm.

An 802.1 μg network will have R_(SensMin)=−94 dBm and R_(SensMax)=−68dBm.

An 802.11a network will have R_(SensMin)=−85 dBm and R_(SensMax)=−68dBm.

Once the coverage metrics F(r₁) and F(r₂) are calculated for the firstand second strongest received signals, a combined coverage metric foreach walkabout point is calculated as follows: coverage=min (1,F(r₁)+F(r₂)). This process is completed for all walkabout points. FIG.7A illustrates coverage metrics for various 802.11 modulation types.

FIG. 9 shows a system block diagram of computer system 900 that may beused to execute software of embodiments of the present invention.Computer system 900 includes memory 902 which can be utilized to storeand retrieve software programs incorporating computer code thatimplements aspects of the invention, data for use with the invention,and the like. Exemplary computer-readable storage media include CD-ROMS,floppy discs, tape, flash memories, system memories, and hard drives.Additionally, a data signal embodied in a carrier wave may be thecomputer-readable storage medium. Computer system 900 further includessubsystems such as central processor 904, fixed storage 906 andremovable storage 908, and one or more network interfaces 910. Othercomputer systems suitable for use with the present invention may includeadditional or fewer subsystems. For example, computer system 900 mayalso incorporate a display for displaying results and/or a keyboard foraccepting input.

The system bus architecture of computer system 900 is represented byarrows 912 in FIG. 9. However, these arrows are only illustrative of onepossible interconnection scheme serving to link the subsystems. Forexample, a local bus may be utilized to connect the central processor904 to the system memory 902. Computer system 900 shown in FIG. 9 isonly one example of a computer system suitable for use with theinvention. Other computer architectures having different configurationsof subsystems may also be utilized.

It is understood that the examples and embodiments that are describedherein are for illustrative purposes only and that various modificationsand changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication and scope of the appended claims and their full scope ofequivalents.

1. A method of assessing communication quality in a wireless networkcomprising a plurality of access points, said method comprising:receiving as input path loss information indicating path losses betweena selected client of said wireless network and said access points; basedon said path loss information, determining a capacity indicator thatestimates communication impairment for said client due to contention orcollision; based on said path loss information, determining a data rateindicator that estimates an achievable data rate for communication bysaid selected client; determining a cell loading indicator thatestimates communication impairment due to overloading of a cell occupiedby said selected client; and based on said capacity indicator, said datarate indicator, and said cell loading indicator, determining a clientthroughput.
 2. The method of claim 1 wherein determining said clientthroughput comprises multiplying said capacity indicator by said datarate indicator and said cell loading indicator.
 3. The method of claim 1further comprising: repeating said receiving, determining a capacityindicator, determining a data rate indicator, and determining a clientthroughput for a plurality of clients; and determining a combinedquality metric as a reciprocal of an average of reciprocals of clientthroughputs determined for said plurality of clients.
 4. The method ofclaim 1 wherein determining a capacity indicator comprises: determininga downstream capacity indicator for an access point associated with saidselected client; determining an upstream capacity indicator for saidselected client; and calculating said capacity indicator as a weightedsum of said downstream capacity indicator and said upstream capacityindicator.
 5. The method of claim 4 wherein said downstream capacityindicator takes into account contention by said associated access pointwith other access points, contention by said access point with clientsother than said selected client, and collision by said associated accesspoint with other access points.
 6. The method of claim 5 wherein saidupstream capacity indicator takes into account contention by saidselected client with access points other than said associated accesspoint and collisions by said selected client with access points otherthan said associated access point.
 7. Apparatus for assessingcommunication quality in a wireless network comprising a plurality ofaccess points, said apparatus comprising: means for receiving as inputpath loss information indicating path losses between a selected clientof said wireless network and said access points; means for, based onsaid path loss information, determining a capacity indicator thatestimates communication impairment for said client due to contention orcollision; means for, based on said path loss information, determining adata rate indicator that estimates an achievable data rate forcommunication by said selected client; means for, determining a cellloading indicator that estimates communication impairment due tooverloading of a cell occupied by said selected client; and means for,based on said capacity indicator, said data rate indicator, and saidcell loading indicator, determining a client throughput.
 8. Theapparatus of claim 7 wherein said means for determining said clientthroughput comprises means for multiplying said capacity indicator bysaid data rate indicator and said cell loading indicator.
 9. Theapparatus of claim 7 further comprising: means for repeating saidreceiving, determining a capacity indicator, determining a data rateindicator, and determining a client throughput for a plurality ofclients; and means for determining a combined quality metric as areciprocal of an average of reciprocals of client throughputs determinedfor said plurality of clients.
 10. The apparatus of claim 7 wherein saidmeans for determining a capacity indicator comprises: means fordetermining a downstream capacity indicator for an access pointassociated with said selected client; means for determining an upstreamcapacity indicator for said selected client; and means for calculatingsaid capacity indicator as a weighted sum of said downstream capacityindicator and said upstream capacity indicator.
 11. The apparatus ofclaim 10 wherein said downstream capacity indicator takes into accountcontention by said associated access point with other access points,contention by said access point with clients other than said selectedclient, and collision by said associated access point with other accesspoints.
 12. The apparatus of claim 11 wherein said upstream capacityindicator takes into account contention by said selected client withaccess points other than said associated access point and collisions bysaid selected client with access points other than said associatedaccess point.
 13. A computer program product for assessing communicationquality in a wireless network comprising a plurality of access points,said product comprising: code that causes receipt of path lossinformation indicating path losses between a selected client of saidwireless network and said access points; code that causes, based on saidpath loss information, determination of a capacity indicator thatestimates communication impairment for said client due to contention orcollision; code that causes, based on said path loss information,determination of a data rate indicator that estimates an achievable datarate for communication by said selected client; code that causesdetermination of a cell loading indicator that estimates communicationimpairment due to overloading of a cell occupied by said selectedclient; code that causes, based on said capacity indicator, said datarate indicator, and said cell loading indicator, determination of aclient throughput; and a computer-readable storage medium that storesthe codes.
 14. The product of claim 13 wherein said code that causesdetermination of said client throughput comprises code that causesmultiplication of said capacity indicator by said data rate indicatorand said cell loading indicator.
 15. The product of claim 13 furthercomprising: code that causes repeated application of said code thatcauses receiving, code that causes determination of a capacityindicator, code that causes determination of a data rate indicator, andcode that causes determination of a client throughput for a plurality ofclients; and code that causes determination of a combined quality metricas a reciprocal of an average of reciprocals of client throughputsdetermined for said plurality of clients.
 16. The product of claim 13wherein said code that causes determination of a capacity indicatorcomprises: code that causes determination of a downstream capacityindicator for an access point associated with said selected client; codethat causes determination of an upstream capacity indicator for saidselected client; and code that causes calculation of said capacityindicator as a weighted sum of said downstream capacity indicator andsaid upstream capacity indicator.
 17. The product of claim 16 whereinsaid downstream capacity indicator takes into account contention by saidassociated access point with other access points, contention by saidaccess point with clients other than said selected client, and collisionby said associated access point with other access points.
 18. Theproduct of claim 17 wherein said upstream capacity indicator takes intoaccount contention by said selected client with access points other thansaid associated access point and collisions by said selected client withaccess points other than said associated access point.
 19. Apparatus forassessing communication quality in a wireless network comprising aplurality of access points, said apparatus comprising: a processor; anda memory device storing instructions for execution by said processor,said instructions comprising: code that causes receipt of path lossinformation indicating path losses between a selected client of saidwireless network and said access points; code that causes, based on saidpath loss information, determination of a capacity indicator thatestimates communication impairment for said client due to contention orcollision; code that causes, based on said path loss information,determination of a data rate indicator that estimates an achievable datarate for communication by said selected client; code that causesdetermination of a cell loading indicator that estimates communicationimpairment due to overloading of a cell occupied by said selectedclient; and code that causes, based on said capacity indicator, saiddata rate indicator, and said cell loading indicator, determination of aclient throughput.
 20. The apparatus of claim 19 wherein said code thatcauses determination of said client throughput comprises code thatcauses multiplication of said capacity indicator by said data rateindicator and said cell loading indicator.
 21. The apparatus of claim 19wherein said instructions further comprise: code that causes repeatedapplication of said code that causes receiving, code that causesdetermination of a capacity indicator, code that causes determination ofa data rate indicator, and code that causes determination of a clientthroughput for a plurality of clients; and code that causesdetermination of a combined quality metric as a reciprocal of an averageof reciprocals of client throughputs determined for said plurality ofclients.
 22. The apparatus of claim 19 wherein said code that causesdetermination of a capacity indicator comprises: code that causesdetermination of a downstream capacity indicator for an access pointassociated with said selected client; code that causes determination ofan upstream capacity indicator for said selected client; and code thatcauses calculation of said capacity indicator as a weighted sum of saiddownstream capacity indicator and said upstream capacity indicator. 23.The apparatus of claim 22 wherein said downstream capacity indicatortakes into account contention by said associated access point with otheraccess points, contention by said access point with clients other thansaid selected client, and collision by said associated access point withother access points.
 24. The apparatus of claim 23 wherein said upstreamcapacity indicator takes into account contention by said selected clientwith access points other than said associated access point andcollisions by said selected client with access points other than saidassociated access point.